The present invention relates to a semiconductor device using a silicon nitride film as an insulating film, and a method of manufacturing the same.
A semiconductor memory device has been widely used as a memory device for use in an information processing apparatus in recent years. The semiconductor memory device is highly resistive to mechanical shock due to the absence of a mechanical driving mechanism. In addition, it is possible for the semiconductor memory to gain high-speed access since a reading-out operation is electrically performed.
However, the tendency toward miniaturization of memory cells, that is, high integration of the semiconductor memory device, has been accelerated by recent progress in semiconductor technologies, particularly in miniaturization technologies. With the high integration of the semiconductor memory device, problems are raised regarding memory cell storage characteristics.
For example, in a DRAM having a memory cell consisting of a MOS transistor and a capacitor connected in series, capacitance is inclined to decrease with reduction in capacitor area due to the high integration. As a result, problems called xe2x80x9csoft errorxe2x80x9d occur. The soft errors are phenomena where a different memory is mistakenly read out, and where a stored memory data is broken by an xcex1ray.
To solve the soft error problems, it is important that the capacitance is maintained even if the memory cell is reduced in size. To attain this, it is necessary to increase a capacitor area as well as to reduce thickness of a capacitor insulating film.
As the capacitor insulating film, a silicon nitride film, which has a higher dielectric constant than a silicon oxide film, has been widely used. The silicon nitride film of this type has hitherto been formed by a low pressure CVD method. However, the silicon nitride film thus formed has a drawback in that a leakage current is likely to increase.
An object of the present invention is to provide a semiconductor device using a silicon nitride film with a low leakage current.
To attain the object, a semiconductor device according to a first aspect of the present invention comprises:
a semiconductor substrate; and
a silicon nitride film formed on the semiconductor substrate, the silicon nitride film being substantially free from an Sixe2x80x94H bond.
The density of Sixe2x80x94H contained in a unit area of the silicon nitride film is 1xc3x971015 cmxe2x88x922 or less.
It is preferable that the silicon nitride film have a film thickness within 4 nm to 8 nm.
The silicon nitride film may have a stacked structure having unit layers of 2 nm or less in thickness stacked one upon another.
The silicon nitride film may be used in at least one of a capacitor insulating film and a gate insulating film.
The silicon nitride film is substantially free from an Nxe2x80x94H bond.
A method of manufacturing a semiconductor device according to a second aspect of the present invention comprises the steps of:
loading a substrate in a low pressure CVD apparatus; and
forming a silicon nitride film substantially free from an Sixe2x80x94H bond by introducing a predetermined silicon source gas into the low pressure CVD apparatus.
The step of forming the silicon nitride film includes a step of forming a silicon nitride film having an Sixe2x80x94H density per unit area of 1xc3x971015 cmxe2x88x922 or less.
A silicon source gas free from an Sixe2x80x94H bond is used as the predetermined silicon source gas.
The predetermined silicon source gas desirably contains silicon whose bonds are entirely bonded to at least one element selected from the group consisting of silicon, nitrogen, and halogen.
The predetermined silicon source gas is preferably one of tetrachlorosilane, hexachlorodisilane, tetrakisdimethylaminosilane and trichlorosilylazide.
The step of forming a silicon nitride film can further comprise the steps of:
forming the silicon nitride film by using the predetermined silicon source gas having an Sixe2x80x94H bond;
reducing hydrogen contained in the silicon nitride film by a heat treatment; and
forming a silicon nitride film having an Sixe2x80x94H density per unit area of 1xc3x971015 cmxe2x88x922 or less.
It is preferable that the heat treatment be performed at a temperature within a range of 900xc2x0 C. to 1100xc2x0 C.
A method of manufacturing a semiconductor device according to a third aspect of the present invention comprises:
a step of loading a semiconductor substrate into a chamber;
a first step of introducing a silicon-containing gas into a chamber;
a second step of performing a first annealing of the semiconductor substrate in the silicon-containing gas at a temperature within 600 to 900xc2x0 C. and a pressure within 0.1 Torr to 10 Torr;
a third step of exhausting the silicon-containing gas after the first annealing;
a fourth step of introducing a nitrogen-containing gas after the third step;
a fifth step of performing a second annealing of the semiconductor substrate in the nitrogen-containing gas at a temperature within 600 to 1000xc2x0 C. and a pressure within 0.5 Torr to 100 Torr;
a sixth step of exhausting the nitrogen-containing gas after the fifth step; and
a step of repeating the first to sixth-steps a plurality of times.
It is desirable that the silicon-containing gas include at least one of tetrachlorosilane, trichlorosilane, and dichlorosilane.
It is desirable that the nitrogen-containing gas include at least one of ammonia, nitrogen trifluoride, hydrazine, dimethylhydrazine and monomethylhydrazine.
A step of thermally nitriding the semiconductor substrate can be included before the first step.
Furthermore, before the first step, it is possible to further include the steps of:
forming a native oxide film on a surface of a semiconductor substrate; and
removing the native oxide film.
Moreover, before the first step, it is possible to comprise the steps of:
forming a native oxide film on a surface of the semiconductor substrate; and
annealing the native oxide film in hexachlorodisilane.
It is desirable that the temperature of the annealing performed in hexachlorodisilane is 400xc2x0 C. or less.
It is preferable that the third and sixth steps include the step of replacing anyone of the silicon containing gas and the nitrogen containing gas with anyone of an inert gas, a hydrogen chloride gas and a hydrogen gas.
It is preferable that an Sixe2x80x94H density per unit area of the silicon nitride film formed in the fifth step is 1xc3x971015 cmxe2x88x922 or less.
The silicon nitride film formed in the fifth step may have a film thickness of 2 nm or less.
The present invention is made based on the following findings.
The silicon nitride film for use in the capacitor insulating film is formed conventionally by a low-pressure CVD method. The silicon nitride film formed by this method contains a large amount of hydrogen (3xc3x971021 cmxe2x88x923 or more).
In this case, hydrogen contained in the silicon nitride film forms an Sixe2x80x94H bond and an Nxe2x80x94H bond. However, since the binding ability of the Sixe2x80x94H bond is especially weak, there are a large number of silicon dangling bonds present in the silicon nitride film. The silicon dangling bond will serve as an electron and hole trap. This means that if the silicon nitride film has a number of silicon dangling bonds, the leakage current will increase.
The leakage current may be reduced by decreasing Sixe2x80x94H concentration so that the number of silicon dangling bonds may decrease. It is found that the Sixe2x80x94H density per unit area of 1xc3x971015 cmxe2x88x922 or less is good enough to reduce the leakage current sufficiently.
According to a semiconductor device of a first aspect of the present invention, the silicon nitride film has an Sixe2x80x94H density per unit area of 1xc3x971015 cmxe2x88x922 or less. Therefore, the silicon nitride film with a low leakage current can be realized.
In the present invention, the thickness of the silicon nitride film is set at 4 nm or more. If the silicon nitride film is formed within the thickness range, it can be formed in a well-controlled thickness.
Furthermore, to reduce the Sixe2x80x94H density per unit area, it is effective to reduce not only Sixe2x80x94H concentration but also film thickness. For example, if the film thickness is set at 8 nm or less as in the present invention, it is possible to form the silicon nitride film satisfying the requirement for the Sixe2x80x94H density of 1xc3x971015 cmxe2x88x922 or less, without difficulties.
In the present invention, it is possible to employ a stacked-form silicon nitride film. The stacked-form silicon nitride film can be easily constructed by repeating a manufacturing method according to a second aspect of the present invention for forming a film of a 2 nm or less in thickness.
The silicon nitride film of the present invention is particularly effective for use in a capacitor insulating film or a gate insulating film, which must be thin (5 nm or less in terms of an oxide equivalent film thickness) and must have a low leakage current.
In the method of manufacturing a semiconductor device according to a second aspect of the present invention, it is possible to form a silicon nitride film having an Sixe2x80x94H density per unit area of 1xc3x971015 cmxe2x88x922 or less by a low-pressure CVD method using a predetermined silicon source gas, that is, a silicon source gas free from an Sixe2x80x94H bond responsible for a silicon dangling bond (trap).
Hence, if the silicon source gas of this type is used, the number of Sixe2x80x94H bonds decreases, resulting in reduction of the dangling bond. Consequently, the leakage current decreases.
In the manufacturing method of the present invention, it is not necessary to form the silicon nitride film with a low Sixe2x80x94H concentration in the beginning. If the formed silicon nitride film is subjected to treatment for reducing the Sixe2x80x94H concentration, it is possible to form the silicon nitride film with an Sixe2x80x94H density per unit area of 1xc3x971015 cmxe2x88x922 or less. More specifically, if the silicon nitride film is subjected to a heat treatment after it is formed, Sixe2x80x94H present in the silicon nitride film can be reduced. Since the number of Sixe2x80x94H bonds is decreased by the heat treatment, the leakage current is reduced.
When the heat treatment is performed at a low temperature, out diffusion of hydrogen rarely occurs. For the reason, the treatment temperature is set at 900xc2x0 C. or more, and preferably at 950xc2x0 C. or more. Whereas, if the treatment temperature is excessively high, other devices are adversely affected. Therefore, the treatment temperature is set at 1100xc2x0 C. or less, and preferably, at 1050xc2x0 C. or less.
According to the manufacturing method of the present invention, a silicon nitride with Si dangling bond is formed and then nitrided. It is therefore possible to terminate the silicon dangling bond with nitrogen. As a result, the silicon nitride film reduced in trap density can be obtained. The leakage current conducting through the trap is reduced.
A silicon nitride film having a region whose Sixe2x80x94H density per unit area is 1xc3x971015 cmxe2x88x922 or less can be formed by varying the nitriding conditions. If so, the leakage current due to the Sixe2x80x94H bond can be further reduced.
If the thickness of the silicon nitride with Si dangling bond is set at 2 nm or less, an entire film can be easily nitrided. Therefore, the leakage current can be effectively suppressed from increasing.
It is important to reduce impurities of the thin nitride film in order to reduce the leakage current from the film. When the nitride film is formed by a conventional CVD method, a large amount of hydrogen and chlorine is inevitably contained in the nitride film, causing current leakage.
In the manufacturing method of a semiconductor device according to a third aspect of the present invention, a silicon source gas and a nitrogen source gas are alternately supplied, with the result that these gases are not mixed with each other. Accordingly, it is possible to suppress a gas-phase reaction. As a result, hydrogen and chlorine are left only on the surface of the film, not in the film. In addition, the reaction proceeds in a self-limitation manner (the reaction amount is limited to a certain degree), the concentrations of hydrogen and chlorine left on the surface can be reduced by extending the heat treatment time sufficiently long.
The reason why the reaction proceeds in a self-limiting manner is that the Si source gas causes a reaction on the surface of the nitride film; however, the reaction rarely takes place on the Si surface at the low temperature without a decomposition of an Si source gas. For example, the study based on a molecular orbital method (shown in FIG. 13) clarifies that since an Nxe2x80x94H bond present on the surface reacts with a tetrachlorosilane, releasing hydrogen chloride, the reaction producing Nxe2x80x94SiCl3 proceeds without energy barrier. Due to this film formation mechanism hydrogen and chlorine contents of the silicon nitride film are reduced and the leakage current can be reduced.
As described above, the semiconductor device of the present invention has a quite good step coverage which is advantageous for permitting the reaction to proceed in a self limiting manner.
When chlorosilane is used as a silicon source, the Nxe2x80x94H bond present on the surface of the nitride film reacts with an Sixe2x80x94Cl bond derived from chlorosilane, thereby removing hydrogen effectively.
When an active nitrogen source such as ammonia or hydrazine is used, chlorine can be easily removed from the Sixe2x80x94Cl bond.
When the treatment is performed at a high-temperature and a relatively high pressure, it is possible to further enhance the effect of removing hydrogen and chlorine. However, when the heat treatment is performed with a silicon-containing gas, the treatment must be performed at a temperature equal to or less than the temperature at which silicon source gas is decomposed. This is because if the heat treatment is performed at an excessively high temperature, silicon will deposit.
Similarly, if the pressure is increased, silicon is deposited due to a gas-phase reaction. Conversely, if the pressure is excessively low, a surface reaction rarely proceeds. Whereas, when the heat treatment is performed with a nitrogen containing gas, it is preferable that the treatment is performed at as high a temperature as possible.
If a substrate surface is thermally nitrided prior to the silicon nitride film deposition step, it is possible to increase the microscopic uniformity in thickness of the silicon nitride film, reducing the leakage current from the silicon nitride film. Particularly, if the native oxide film is thermally treated with hexachlorodisilane, a silicon nitride film can be formed on an extremely thin oxide film, in a low interface trap density and more uniform thickness than that formed by a conventional thermal nitriding method.
After completion of a predetermined treatment, each of source gases is replaced with an inert gas, hydrogen gas or a hydrogen chloride gas, thereby suppressing a reaction of each source gas in a gas phase. As a result, the bonds containing impurities, such as Sixe2x80x94Cl, Sixe2x80x94H, and Nxe2x80x94H, are formed only in the uppermost surface. The compounds thus formed can be easily removed. As another advantage of the present invention, since the gas-phase reaction is suppressed, the dust generated during the film deposition step is successfully reduced.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.